Pediatric emergency medicine trisk 2878 2878

of CO2 generated. Once metabolic acidosis is diagnosed, assessment of
respiratory compensation can be performed. The arterial partial pressure of
CO2 (pCO2 ) normally falls at an average of 1.2 mm Hg for every 1 mEq/L
reduction in the serum HCO3 − to a minimum pCO2 of 10 to 15 mm Hg.
The Winters formula can also approximate the expected arterial pCO2 :
Arterial pCO2 = (1.5 × measured HCO3 − ) + 8 ± 2
If the fall in pCO 2 is significantly different than the expected value, a
mixed acid–base disorder is likely present.
Patients with metabolic acidosis can be classified in two groups based on
the serum AG, and this determination may facilitate the diagnosis of the
underlying cause. Serum electrolytes should be obtained to calculate the
AG using the following formula:
AG (mEq/L) = Na+ − (Cl− + HCO3 − )
The normal AG typically ranges between 7 and 13 mEq/L and an AG
greater than 14 to 16 mEq/L is elevated. In normal subjects, the AG is
primarily due to negative charges from plasma proteins, particularly
albumin. An elevated AG is most often due to an unmeasured anion, though
can also result from a low concentration of serum cations (K+ , Ca2+ , Mg2+
). A low AG can result from an increase in serum cations or low serum
albumin. For every 1 g/dL reduction in serum albumin level, the AG would
be expected to be reduced by approximately 2.5 mEq/L. The causes of
metabolic acidosis based on elevated or normal AG are outlined in Table
100.11 . Potentially life-threatening conditions including septic shock,
severe hypovolemia, and certain inborn errors of metabolism are important
considerations in the evaluation of children with significant elevated AG
metabolic acidosis. DKA is associated with an elevated AG due to the
accumulation of acetoacetic acid and β-hydroxybutyric acid. Lactic acidosis
has been observed in patients with severe asthma being treated with highdose β-adrenergic agents and steroids. Some toxic ingestions, including
ethanol, methanol, ethylene glycol, other alcohols, and salicylates, are
associated with unmeasured serum osmoles. This leads to an elevated